Some luminescent substances (phosphors), which comprise an inorganic compound (host) activated with element ions (activators), emit light (which is usually visible light but may be also infrared light and it is generally called soft or low energy radiation) upon exposure to hard radiation or high energy radiation, such as x-rays, cathode rays, UV rays.
The soft radiation emission may be either a direct emission, i.e. the phosphor emits light while it is still under irradiation, or a photostimulated emission, i.e. the irradiated phosphor emits light if stimulated by a suitable stimulation light.
These phosphors have different applications. In particular, direct emission phosphors are used in cathode ray tubes (CRT), converting screens for direct radiography, and converting linings for UV lamps. Photostimulated emission phosphors are used in storage panels for digital radiography and in dosimetry.
The intensity of the soft radiation emitted by a phosphor is not the same throughout the light spectrum; each phosphor has its own particular emission spectrum, which comprises one or more peaks and/or bands which are typical of that phosphor.
The performance of a phosphor is generally evaluated by its luminescence, which is the rate between the intensity of the emitted light and the intensity of the hard radiation; it may be evaluated as an overall luminescence (in which case the intensity of the emitted light throughout the whole light spectrum is considered) or as band luminescence (in which case the intensity of the light emitted in a number of bands of the spectrum is considered).
Generally speaking, the overall luminescence is a significant parameter for CRT application, since the emitted light is to be "detected" by human eyes. On the contrary, for radiography applications the relevant luminescence is that in the band of higher sensitivity of either the photographic film (direct radiography) or the photodetector (digital radiography) which are coupled to the phosphor screen or panel for detecting the emitted light.
Therefore, the general task in this field is to find more efficient phosphors, i.e. having the highest possible luminescence.
Among direct emission phosphors known in the art, the most efficient are generally considered the gadolinium oxy-sulfide activated with terbium (Gd.sub.2 O.sub.2 S: Tb), the barium fluoro-chloride activated with europium (BaFCl:Eu), the calcium tungstate (CaWO.sub.4), the yttrium tantalate activated with niobium (YTaO.sub.4 :Nb), the gadolinium tantalate activated with terbium (GdTaO.sub.4 :Tb).
All these phosphors emit in particular in the blue or blue-green light region.
Other direct emission phosphors are known from the patent literature, such as the phosphors obtained with the following raw materials (after mixing, adding a flux, burning, grinding, washing, drying, sieving, etc): yttrium oxide, silicon dioxide, terbium oxide, dysprosium oxide or praseodimium oxide, barium oxide (see Japanese patent application Ser No. 84-193983); these phosphors should have an yttrium/dysprosium or yttrium/praseodimium silicate structure, even if this fact is not sufficiently clarified in the above mentioned patent application.